In integrated-services networks, it is difficult to provide adequate quality of service to large bandwidth calls, such as video conference calls, without adversely affecting the network utilization. One way to alleviate this problem is to allow customers of the network to book their calls ahead of the actual call initiation time, much like calling ahead to make a reservation at a restaurant.
It would be possible to allow for both book-ahead calls (which announce their expected holding times) and instantaneous-request calls (which do not) by partitioning the network resources into two disjoint subsets dedicated to each class. Alternatively, the network resources could be shared without strict partitioning by, e.g., having a moving boundary between the classes, thereby allowing resource sharing without allowing any calls in progress to be interrupted.
There is a drawback, however, to using strict resource partitioning schemes which do not allow call interruption. Such resource sharing schemes often lead to under-utilization of network resources. It may be desirable, therefore, to implement resource sharing in a manner that allows some calls in progress to be interrupted, but increases network resource utilization and the revenue derived therefrom.